util: add keyring_describe helper and move to basic

So that it can be used from libsystemd. No external dependencies.

src/basic/keyring-util.h

@@ -0,0 +1,12 @@
+/* SPDX-License-Identifier: LGPL-2.1-or-later */
+#pragma once
+
+#include <sys/types.h>
+
+#include "missing_keyctl.h"
+
+/* Like TAKE_PTR() but for key_serial_t, resetting them to -1 */
+#define TAKE_KEY_SERIAL(key_serial) TAKE_GENERIC(key_serial, key_serial_t, -1)
+
+int keyring_read(key_serial_t serial, void **ret, size_t *ret_size);
+int keyring_describe(key_serial_t serial, char **ret);